In the field of parts for connecting electric cables or the like, EPDM (crosslinked rubber of ethylene-propylene-diene ternary copolymer) is frequently used. This is attributable to a recovery property after tension applied thereto, a rubber property such as flexibility, an insulating property, an electric property such as a tracking resistance property, a heat resistance property, a long-term property such as heat aging resistance, and such a superior property as a high degree of freedom for a composition for the sake of corresponding flexibly to levels demanded for the above properties. These EDPMs are produced by a crosslinking process through processes using peroxides and sulfuric compounds together (this process is generally called as “vulcanizing process” in rubber). This vulcanizing process requires a long time in manufacturing to lower its productivity, causing its manufacturing cost to be substantially pushed up.
In order to solve these problems, a study for manufacturing products of EDPM by a mold injection process is on-going and the manufacturing is put to practical use. Nevertheless, the vulcanizing process inside a metallic mold is still needed. There is further a problem that when manufacturing a multi-layered molded object, an interlayer adhesive property is poor. Hence, the improvement in productivity is limited since a surface treatment should be carried out for every layer to need to improve the adhesive property and so on. Further, there is a problem that a composition study for improving fluidity such as adding a developing oil in the composition must be newly performed. In the field of the parts for connecting the electric cables, e.g., there is a case where properties (an electric property) required for the products are impaired, and a phenomenon such as the oil transferring to other layers occur, thus causing a new problem in reliabilities of processing and products.
As a resolution method for these problems, it is considered that a thermoplastic elastomer, e.g., is used. As substitute for EPDM, a polyolefin-based thermoplastic elastomer is most suitable and the elastomer, which does not need the vulcanizing process differently from EDPM and has such a grade of commercial products as is usable for any one of extrusion, injection, and molding methods, is available in the market. A polyolefin-based thermoplastic elastomer material is classified roughly into two types from its makeup of components. One type includes a crosslinked body of rubber acting as a dispersion phase in a thermoplastic resin serving as a matrix. The other type is an olefin-based copolymer whose crystallinity is intendedly lessened. In both the elastomer materials, however, their physical properties widely vary near their melting points and besides a plastic property remains at a normal operating temperature range. Therefore, since there are some cases where their recovery actions are poor after deformation, a necking phenomenon occurs after large deformation and so on, both the elastomer materials cannot exert a property suitable to that of rubber, so that there exists no elastomer material with a satisfactory property as substitute for rubber.
In order to make up for such a lack of the rubber property, the crosslinking process is on-going for the elastomer. There are generally three crosslinking methods including a peroxide crosslinking one, a silane crosslinking one, and a radiation crosslinking one (see a patent document 1). However, the crosslinking method using peroxide is the same method as that for manufacturing EDPM and the manufacturing flow involves the crosslinking process, and therefore the purpose for improving the productivity cannot be achieved. Further, although the crosslinking method using radiation is advantageous from the viewpoint of capable of readily obtaining a crosslinked body, a radiation irradiating system is extremely high in price to considerably push up the manufacturing cost of the crosslinked body, thus becoming unsuitable to practical use.
It is known that in the silane crosslinking method, a process for carrying out a silane modification of the thermoplastic elastomer while maintaining a suitable rubber property is extremely difficult. This is because a polyolefin-based thermoplastic elastomer excellent in rubber property is easy to be subjected to radical decomposition and hence the resin in which a crosslinkability and the rubber property are compatible is hard to obtain. The reason for this is that the olefin-based thermoplastic elastomer commonly known is not composed of a single component, but there are many elastomers blended with heterogeneous components. As cited above, as a representative one, there is an elastomer material using a resin (polypropylene) easy to decompose as a matrix and containing, in a dispersion phase, a crosslinkable resin such as crosslinked EPDM. At the time of manufacturing, such an elastomer is allowed to undergo a crosslinking reaction and a decomposing reaction to develop the rubber property in a delicately-balanced state. Accordingly, when such an elastomer further undergoes the silane modification process, a physical-property balance is broken down and hence a crosslinked body meeting a condition of being ought to be a substitute for EDPM cannot be obtained.
Then, when using, as a thermoplastic elastomer, an ethylene-propylene copolymer whose propylene component is increased for the purpose of lowering a degree of crystallinity, if achieving an ideal crosslinking condition, the ethylene-propylene copolymer is optimal as a substitute. There are, however, plenty of decomposable propylene components therein and hence this copolymer is considerably decomposed in a process for applying a graft reaction to a silane coupling agent to become thereby unable to maintain the rubber property. For this reason, a physical property substitutable for EPDM is extremely difficult to develop. Accordingly, when trying to manufacture a superior silane crosslinked body using such a common thermoplastic elastomer, the resin itself needs to review, resulting in a high material cost. This is why there exists no low-price commercialized products under the present circumstances.
Also, International Patent Application Publication No. WO 2010/009024 discloses such an ethylene-propylene copolymer, excellent in a heat shielding performance, subjected to the radiation crosslinking process. However, a silane-crosslinked ethylene-propylene copolymer which develops a superior rubber property and is subjected to a silane crosslinking process is not disclosed therein.